A critical performance measure in image projection systems is the contrast ratio (C/R) which represents the light intensity difference between the brightest white and the darkest black. C/R is thus defined by the relationship:C/R=white luminance/black luminance  (1).Improving the C/R helps to provide better on-screen image reproduction.
In general it is difficult to increase C/R by increasing the white luminance of the projection system since most projectors are light source limited. Thus approaches to improve the contrast ratio attempt to decrease the black luminance level. The black luminance level is a result of light that passes through the active display devices in the off state and stray light coming through the projection optics of the display. Stray light can come from unwanted reflections from optical components of the projection system.
There have been a number of proposed approaches to improving the contrast ratio of an electronic display. For example, in U.S. Pat. No. 7,413,314 (Kim et al.) describes an optical system having an iris controlled in real time for reducing light from devices in the off state. In the '314 optical system, an iris controller senses luminance information in the light output and controls the projection iris according to the luminance information. With the opening range of the iris controlled in real time, the contrast ratio (C/R) is improved.
U.S. Pat. No. 7,204,594 (Akiyama) describes a projector including an illumination device, an electro-optic modulator, and a projection optical system that includes a light shielding member provided with a stray light elimination member that reflects unwanted light away from the projection optics path.
Another approach has been to modify the display screen itself. For example, U.S. Pat. No. 7,403,332 (Whitehead et al.) describes a display having a screen incorporating a light modulator which is illuminated by a light source composed of an array of controllable light emitters. The controllable emitters and elements of the light modulator may be controlled to adjust the intensity of light emanating from corresponding areas on the screen.
Each of these conventional approaches for contrast ratio improvement has its shortcomings. The mechanical iris of the '314 disclosure must be a high-speed device and can be relatively costly. The light-shielding member of the '594 disclosure sends stray light out of the projection path and to other surfaces inside the projector, with the potential for some portion of this light to be projected onto the screen. The specialized display screen taught in the '332 disclosure adds significantly to projection system cost and may not be a suitable solution where it is desirable to replace existing film projection equipment.
Of particular interest are solutions that are appropriate for projection systems that use laser light sources. These can include, for example, systems that use spatial light imaging modulators such as liquid crystal devices (LCDs) or digital micromirror devices, such as the DLP device from Texas Instruments, Inc., Dallas, Tex.
Another type of imaging modulator device that is well-suited for use with laser sources are linear light modulators. Linear light modulators form images by a rapid, repeated sequence in which each single line of the image is separately formed and is directed to a screen or other display surface by reflection, or other type of redirection, from a scanning element. Types of linear light modulators that operate in this manner include devices such as grating light valves (GLV) from Silicon Light Machines and described in U.S. Pat. No. 6,215,579 (Bloom et al.), and elsewhere. Display systems based on GLV devices are disclosed, for example, in U.S. Pat. No. 5,982,553 (Bloom et al.). Another type of linear light modulator is the piezoelectric based spatial light modulator (SOM) developed by Samsung and disclosed, for example, in U.S. Pat. No. 7,133,184 (Shin et al.).
An improved type of linear imaging modulator is the grating electro-mechanical system (GEMS) device, as disclosed in commonly-assigned U.S. Pat. No. 6,307,663 (Kowarz), and elsewhere. Display systems based on a linear array of conformal GEMS devices are described in commonly-assigned U.S. Pat. Nos. 6,411,425, 6,678,085, and 6,476,848 (all to Kowarz et al.). Further detailed description of GEMS device architecture and operation is given in a number of commonly-assigned U.S. patents and published applications, including U.S. Pat. Nos. 6,663,788 (Kowarz et al.), and 6,802,613 (Agostinelli et al.). In these devices, light is modulated by diffraction. On a GEMS chip, for example, a linear array of conformal electromechanical ribbon elements, formed on a single substrate, is actuable to provide one or more diffracted orders of light to form each line of pixels for line-scanned projection display.
Color display system architectures using LCD, DLP, GLV, SOM, and GEMS devices generally employ three separate color paths, red, green, and blue (RGB), each color path provided with a separate spatial light modulator and laser source. When actuated, the light imaging modulator modulates its component red, green, or blue laser light to form the image, a single frame of pixels or line of pixels at a time. The resulting modulated frames of pixels or lines of pixels for each color are then combined onto the same output axis to provide a full-color image that is then directed onto the display screen.
Linear light imaging modulator arrays have exhibited some advantages over their area array spatial light modulator (SLM) counterparts by virtue of higher resolution, reduced cost, and simplified illumination optics. GLV and GEMS devices are actuable to operate at fast switching speeds for modulating laser light. GLV and GEMS devices have advantages for high resolution, high native bit depth, variable aspect ratio, and relative freedom from motion artifacts when compared against other types of spatial light modulators.
However, there are a number of limitations inherent to linear spatial light modulators that can tend to constrain projector performance. A number of limitations relate to the scanning sequence itself. The galvanometrically actuated scanning mirror that is conventionally used to scan modulated light across the display surface rotates over a short angular range to form each 2-D (two-dimensional) frame of the image. Following each scan, mirror position must then be reset into the starting position for the next scan. During this reset interval, image content is not projected, when using the standard scanning sequence. Thus, light output is not available during about 15-25% of the operating cycle, since the mirror requires some amount of time to stop, reverse direction, and return back into position for the next scan. This inherent reduction of the available light output limits the light efficiencies that can be obtained. Due to this scanning mirror reset time and to acceleration and deceleration times of the mirror, the effective duty cycle for providing modulated light with such systems, the so-called “pixel on” time, is typically no more than about 72-85%.
Another problem related resulting from the scanning sequence relates to the need to minimize the effects of stored charge as the ribbon elements are repeatedly switched between positions. Electrostatic energy is used to actuate the ribbons. Maintaining the same charge polarity for the integrated circuit (chip) substrate from one scan to the next quickly builds up a residual charge in the device that must be compensated for or dissipated in some way. In response to the problems of charge build-up, commonly-assigned U.S. Pat. No. 6,144,481 (Kowarz et al.) discloses a method for correcting for charge accumulation in the spatial light modulator device. This method applies, to the dielectric ribbon elements, a modulated bipolar voltage signal whose time average is equal to the time average of a bias voltage applied to the bottom conductive layer of the modulator device. The resulting alternating waveform switches the polarity of the substrate bias voltage effectively canceling the charge build-up during operation of the device.
Although the method described in the Kowarz et al. '481 disclosure corrects for problems related to charge build-up, transient movement of the modulating ribbon elements can result as the voltage is switched. Usually the voltage is switched during the reset interval of the scanning mirror and stray light can reach the screen when the voltage is switched, thus degrading system contrast. A small amount of light is also inadvertently directed into the optical system during this transient which can result in extra reflections and stray light passing through the projection optics reaching the display screen. All of these factors can degrade system contrast.
Area spatial light imaging modulators such as DLP devices do not exhibit the same switching effects as linear GEMS, SOM, and GLV devices. However, both area and linear light-modulating devices have a refresh cycle, during which unmodulated light can be inadvertently directed to the display surface. While the laser itself could be momentarily turned off to eliminate stray light during the refresh cycle, such a mode of operation is not optimal for existing semiconductor laser devices, compromising wavelength and thermal stability and potentially shortening laser lifetimes.
Related to the problem of image contrast is the relative distribution of data values over portions of the image. Many types of images include an area or band of darker values over which there is little perceptible difference in intensity. Stated differently, such images exhibit a high percentage of relatively indistinguishable dark values or “dark noise.” Because there is little difference in contrast over such an area, many features within the image are effectively lost. The skyline of FIG. 1 shows one example. In this image, only a silhouette of the skyline is clearly displayed; there is little or no perceptible detail within the skyline band. Buildings, for example, are seen substantially in outline, with almost none of the features within the outline of a building visible. Using conventional display and data value mapping techniques, this poor contrast over such a local area is the best that can be achieved. Even with imaging equipment that is sensitive enough to capture subtle differences in detail, areas of poor contrast over darker regions of the image effectively prevent this detail from being displayed.
Thus, it can be seen that improving image contrast relates not only to methods that help to reduce stray light, but also to methods that can help to make details more visible within darker areas and other local areas of an image. There is a need to simultaneously decrease stray light and to improve the contrast of objects in darker areas of an image in projection apparatus where laser illumination is employed.